In a typical hydraulic elevator, a car is raised and lowered by a plunger/cylinder assembly located beneath the car. The assembly comprises a plunger that translates upwardly and downwardly within a cylinder. A seal is provided at an interface between the cylinder and plunger to prevent leakage of hydraulic fluid while allowing the pressure force of the fluid to act upon the plunger.
The seal should allow the plunger to translate without experiencing drag. However, elevators experience a phenomenon known as "stick-slip" in which the plunger sticks to the seal when the elevator car is stopped and suddenly slips when the sticking friction is broken. Upon start-up of the system, a pressure gradient in the hydraulic fluid urges the plunger to move until the sticking friction between the plunger and the seal is broken forcing the car to rise or fall suddenly. Stick-slip is usually experienced by elevator passengers as a sudden acceleration or bump at the beginning of elevator travel.
Several methods have been tried to reduce the effect of stick-slip. Among them are special materials, special surface finishes, and special seal section shapes. None have been totally effective.
Another problem hydraulic elevators encounter relates to stopping a car when it overruns terminal stopping devices and engages physical stops. Elevator codes require that there be a means to stop the upward travel of an elevator moving at contract speed when limit switches fail. Typically, the plunger has a stop ring welded on a bottom portion thereof and the cylinder has an internal ring or shoulder disposed within a top portion thereof. If the stop ring engages the internal ring abruptly, there is a great potential, because of the forces involved, that either ring could be damaged.